This invention relates generally to audio scrambling and unscrambling systems and particularly to a two channel audio scrambling and unscrambling system for use in conjunction with a pay television signal encoding system.
Pay television systems, which are normally operated in connection with cable connected television subscription services, have grown in popularity and have given rise to a need for added security to restrict certain programming to those who have specifically subscribed and paid for it. Common techniques for television signal encoding include suppressing the horizontal sync pulses and randomly inverting the video information. The audio portion of the program may also be encoded by a number of well known techniques.
A copending application of two of the co-inventors of the present application, entitled "Dynamic Audio Scrambling System", Ser. No. 564,973, filed Dec. 21, 1983 in the names of J. Lund and R. Forbes and assigned to Zenith Radio Corporation, describes a prior art audio encoding/decoding scheme involving modulating audio information with a small offset frequency to displace the entire audio information spectrum upwards in frequency and the improvement in randomly changing the offset frequency to enhance system security.
There is a perceived need in pay television systems for stereo sound, that is audio information that is derived from two related audio channels. The technique of the copending application can be used for stereo sound, or for two channel sound, by simply doubling the system. Such an approach would surely involve substantially higher costs and an increase in bandwidth. The present invention provides the enhanced security of the system of the above application with stereo signals.